The technology giant, which has faced criticism over working conditions at some of its suppliers’ plants in China, has asked the Fair Labor Association (FLA) to conduct “special voluntary audits” of several facilities, including factories owned by Foxconn, the world’s largest electronics manufacturer, in Shenzen and Chengdu.

A team of inspectors from the not-for-profit organisation, which is headquartered in Washington DC, started inspections in February at the Foxconn City plant in Shenzen.

Apple’s problems with Taiwanese company Foxconn, which manufactures almost all of its devices, date back as far as 2010 when a string of workers committed suicide at a plant in Longhua, which employed between 300,000 and 400,000 workers.

However the troubles have continued and in January, 150 Foxconn employees threatened to leap from the top of a three-floor plant in Wuhan amid allegations they were paid piecemeal and were expected to work in a pressured environment without any training.

The FLA has already reported backed that it has found “tons of issues” but has also seen some “dramatic improvements”.

It is not known how long Cook spent at the plant, nor who he spoke to while he was there.